Field of the Invention
The present invention relates to a water base ink for ink-jet recording.
Description of the Related Art
In the ink-jet recording, ink droplets are formed by using the ink discharge method including, for example, the electrostatic attraction method, a method in which mechanical vibration or displacement is applied to the ink by using a piezoelectric element, and a method in which bubbles are generated by heating the ink to utilize the pressure generated thereby, and all or a part of the ink droplets are adhered to a recording medium such as paper.
In recent years, the ink-jet recording method rapidly comes into widespread use, because the ink-jet recording method involves, for example, neither development process nor fixation process, and it is easy to effect the color printing. Nowadays, the high-definition printing and the high speed printing are rapidly advanced and realized by using the ink-jet printer. Further, for example, the color printer, which is capable printing on regular paper, is dominantly used.
In such a situation, high performance is required for the ink for ink-jet recording to be used for the ink-jet recording method as well. For example, the following performance is required. That is, the stable jetting operation can be performed without causing any clog-up at the tip of the head of the ink-jet printer and in the ink flow passage. A recorded image is given, in which the color tone is vivid and the density is sufficiently high.
In general, the ink, which is used for the ink-jet recording method, is roughly classified into the oil base ink and the water base ink. In the case of the oil base ink, the wettability is satisfactory with respect to the inner wall of the head of the ink-jet printer, and it is possible to perform the stable jetting operation. However, the oil base ink involves problems concerning odor and toxicity. At present, the water base ink is dominantly used.
In the case of the ink-jet printer which uses the general water base ink, the viscosity of the ink, at which the jetting operation can be normally performed, is about 1 to 10 mPaxc2x7s. However, when the volatile components such as water in the ink are evaporated, then the viscosity of the ink is increased at the nozzle tip of the ink-jet head, and the ink is consequently dried-up. As a result, the nozzle is clogged in some cases, and/or the jetting direction is deflected in other cases. In order to avoid the inconveniences as described above, a method is generally adopted, in which the jetting orifice of the head is covered with a nozzle cap in a tightly closed state during the period in which the jetting operation is not performed. This method functions sufficiently effectively under the ordinary condition of use.
However, when the ink is left to stand without effecting any tight closure with the nozzle cap or the like for several days or for a long period of time longer than the above in a state in which the nozzle disposed at the head tip of the ink-jet printer is filled with the ink, the ink, which is located on the nozzle surface, undergoes considerable increase in viscosity caused by the evaporation of the volatile components. In the worst case, any deposit appears, and the ink is consequently dried-up. As for the ink having been in the state of the increase in viscosity, the appearance of deposition, and the occurrence of drying-up as described above, it has been almost impossible that the ink is restored to give the normal state by means of a conventionally known purge device in which the ink is forcibly discharged from the ink-jet head by using a pump or the like.
The present invention has been made in order to solve the problem as described above, an object of which is to provide a water base ink for ink-jet recording which exhibits satisfactory jetting stability during the normal use and which is easily restored to give a normal jetting state even under an abnormal condition in which the ink is left to stand without any nozzle cap, and an ink-jet recording apparatus which accommodates the ink.
As a result of diligent investigations performed by the present inventors in order to achieve the object as described above, the present inventors have empirically found out that the nozzle restoration performance is satisfactory when the amount of water in an ink and the saturation viscosity satisfy a certain relationship for the ink prepared by combining a variety of constitutive components and compositions. Thus, the present invention has been achieved.
According to a first aspect of the present invention, there is provided an ink for ink-jet recording, which contains water, wherein an amount of water X (% by weight) in the ink and a saturation viscosity Y satisfy the following expression (1):
Yxe2x89xa64.5e0.09Xxe2x80x83xe2x80x83(1) 
wherein the saturation viscosity Y is a viscosity (mPaxc2x7s) obtained when an ink residual liquid, which is obtained by evaporating volatile components of the ink until a change in weight of the ink is less than 0.05 g/day per 50 g of ink under a condition of a temperature of 60xc2x0 C. and a humidity of 40%, is cooled to a temperature of 25xc2x0 C. in an environment of a humidity of 40%, and X satisfies 50xe2x89xa6Xxe2x89xa675.
In the water base ink for the ink-jet recording according to the first aspect of the present invention, the amount of water X in the ink and the saturation viscosity Y satisfy the relationship represented by the foregoing expression (1). Accordingly, the water base ink for ink-jet recording exhibits satisfactory jetting stability during the ordinary use. Further, the water base ink is easily restored to give a normal jetting state even under an abnormal condition, for example, when the ink is left to stand without any nozzle cap. If a water base ink for ink-jet recording, which does not satisfy the relationship represented by the foregoing expression (1), is used, it is impossible to make restoration to the normal state with a purge device, when the viscosity is once extremely increased, when any deposit appears, or when the ink is dried up, as a result of the evaporation of volatile components of the ink disposed on the nozzle surface. In the present invention, for example, taking the stable discharge and the durability into consideration, it is preferable that the saturation viscosity Y satisfies the following expression (2):
Yxe2x89xa62.0e0.09Xxe2x80x83xe2x80x83(2) 
The ink may further comprise a water-soluble organic solvent and a coloring agent. The amount of water in the ink may be 55 to 73% by weight. The ink may contain a water-soluble organic solvent by 10 to 45% by weight and a coloring agent by 0.1 to 5.5% by weight.
According to a second aspect of the present invention, there is provided an ink-jet recording apparatus comprising an ink-jet head; an ink tank which accommodates an ink to be supplied to the ink-jet head; and the ink according to the first aspect of the present invention which is accommodated in the ink tank. The ink tank may be a replaceable ink cartridge or an ink container which is fixedly provided in the ink-jet recording apparatus.